1. Field of the Invention
This invention relates to carrier complexes for use in apparatus and methods for extracting molecular oxygen from a fluid; more particularly, the invention relates to electrochemically active polyamine (also referred to as polyalkylamine) complexes of transition metals that reversibly bind small ligands such as molecular oxygen and the use of such complexes for extraction of ligands from a first fluid environment and release of ligands to a second fluid environment.
2. Relevant Art
Molecular oxygen is used in many industrial, scientific, medical, and recreational applications.
A wide variety of methods for extracting oxygen from air, sea water, and other fluids are known. On a large scale, oxygen is generally prepared by fractional distillation of liquid air. Typically, filtered air is passed through an alkali absorbent in order to remove moisture and carbon dioxide. The air is then compressed, and the heat of compression is removed by ordinary cooling procedures. The cooled compressed air is allowed to expand which causes it to cool further. The expanded air is then recompressed, cooled, and reexpanded repeatedly to liquify the air. Liquid air is then fractionally distilled to remove nitrogen and other impurity gases. The remaining liquid oxygen may be stored in that form or as compressed gaseous oxygen. Cryogenic methods of oxygen production typically entail large installations which are not portable and are energy intensive. The production and storage of oxygen present severe explosion and fire hazards. While cryogenic fractional distillation processes have proved useful for supplying oxygen to large industrial users, such as the steel industry and sewage treatment plants, there are many applications in which local production of oxygen in relatively smaller quantities is both useful and desirable.
Numerous methods and devices for separating oxygen from fluids such as air and sea water have been devised. Many utilize gas permeable membranes to separate oxygen from the particular fluid feedstock by diffusion. For example Bodell, U.S. Pat. No. 3,333,583, and Robb, U.S. Pat. Nos. 3,369,343, and 3,510,387, disclose apparatus for extracting oxygen from sea water using thin tubes of silicon rubber or a membrane of silicone rubber, respectively. Isomura, U.S. Pat. No. 3,377,777, discloses concentrating oxygen from natural waters by equilibration with exhaled gases i.e., by utilizing large areas of gas water interface and simple diffusional considerations for transport of oxygen from the liquid phase across the permeable membrane to a gas phase. Systems based on simple diffusion processes are also taught by: Blanchard, et al., U.S. Pat. No. 3,651,616; Blackmer, et al., U.S. Pat. No. 3,976,451; Hedman, U.S. Pat. No. 3,979,190; and Shindo, et al., U.S. Pat. No. 4,268,279. Such systems have proven impractical for actual use due to a variety of factors including: the lack of selectivity of the membranes for oxygen, the size or weight of the system required to produce useful quantities of oxygen, and the limitation that the maximum partial pressure of oxygen which can be extracted without additional vacuum or compression pumps is approximately equivalent to that of the fluid feed stock.
The permeation rates and selectivity of such membranes can be increased by coating the delivery side of the membrane with a gas-solubilizing substance (Allington, U.S. Pat. No. 3,751,879) or by incorporating transition metal-based carrier complexes in the membrane (e.g., Nishide, et al., Macromolecules 19, 494-496 (1986)). Other examples of facilitation of the rate or enhancement of the selectivity of diffusion extraction by incorporation of absorbing materials in polymeric membranes are given by Ward, et al., U.S. Pat. No. 3,396,510. While the use of such membranes would be an improvement in oxygen extraction devices, the partial pressure of oxygen that may be delivered remains limited by the effective partial pressure of oxygen in the feed stock fluid.
A second type of oxygen source that is portable and can supply oxygen in small amounts as needed is disclosed by Rind, U.S. Pat. No. 4,020,833. This system includes a mixture of a metallic superoxide, decomposable to release oxygen upon contact with carbon dioxide and water vapor, and a material which absorbs CO.sub.2. The utility of this system is limited in that the capacity of oxygen which can be produced is limited by the bulk of material which can be carried. In addition, the supply of chemical reactants must be continuously replenished for continued use of oxygen.
A variety of electrochemical methods of producing oxygen from water have been developed. The most basic of these methods is the electrolysis of water into component hydrogen and oxygen gases. Where oxygen is the product of primary interest, electrolysis techniques usually suffer from the significant disadvantage of cogenerating hydrogen. The oxygen produced is typically contaminated with small quantities of hydrogen. In addition, the hydrogen produced presents a severe fire or explosion hazard which makes such systems generally unsuitable for use in closed environments and nearby people. Electrolysis of water to produce oxygen is extremely energy intensive. Some of these disadvantages have been ameliorated by the apparatus disclosed by Nolan, U.S. Pat. No. 4,488,951, in which the hydrogen produced from an electrolysis cell is electrochemically reacted with oxygen from air to form water and hydrogen peroxide; the hydrogen peroxide produced is then decomposed to form water and oxygen. However, the power requirements of such a cell remain quite large.
Other electrochemical methods of oxygen production utilize a variety of two- and four-electron redox processes to extract oxygen from air. Tseung, et al., U.S. Pat. No. 4,416,758, describes a cell in which oxygen from air is reduced in solution at a graphite electrode to form peroxyl and hydroxyl ions. The peroxyl ions so produced diffuse through an ion-permeable membrane and are catalytically reacted to form oxygen in a second cell compartment with a NiCo.sub.2 O.sub.4 or CoFe.sub.2 O.sub.4 catalysts. This cell, which requires an operating voltage of about one volt, uses a concentrated alkaline electrolyte at elevated temperatures and consumes substantial electrical energy. Chillier-Duchetel, et al., U.S. Pat. No. 4,061,445, also regenerates oxygen from peroxyl ions. The cell uses bipolar electrodes coated with anthraquinone derivatives; these anthraquinones are electro-chemically reduced and, in turn, reduce the oxygen to the desired peroxyl ions. The peroxyl ions are reacted at the anode to regenerate oxygen gas. Blanchard, et al., U.S. Pat. No. 4,137,371, discloses a zinc oxide cell which extracts oxygen from the air by diffusion, Gagne, et al., U.S. Pat. No. 4,475,994, discloses an electrochemical cell in which oxygen from the air is reduced to superoxide ions (O.sub.2.sup.-) on a quinoline-coated electrode. The superoxide anions are transported to the anode using transition metal carrier compounds. At the anode, the superoxide is reoxidized to oxygen. Tomter, U.S. Pat. No. 3,410,783, discloses electrolytic reduction of air-borne oxygen at one electrode and regeneration of oxygen at a second. The reduced oxygen species are pumped or diffuse, respectively, between the two electrodes.
It has long been known that the variety of naturally occurring metalloproteins, including hemoglobin. myoglobin, hemocyanin, and hemerythrin, are capable of reversibly binding oxygen and transporting oxygen from a permeable membrane to a site within an organism at which the oxygen is needed. In hemoglobin, for example, oxygen is reversibly bound to ferrous (Fe(II)) porphyrins incorporated in the protein. Oxidized, ferric hemoglobins are unreactive to molecular oxygen. The properties of hemoglobins, hemerythrins, and hemocyanins have been the subjects of numerous studies, as documented in, e.g., Bonaventura,. et al., Symposium on Respiratory Pigments, J. Am. Zool. 20, 7 (1980) and 20, 131 (1980).
The oxygen binding properties of such proteins have been utilized to extract oxygen from air and other fluids. Miller, U.S. Pat. No. 3,230,045, discloses the use of an oxygen-binding chromoprotein such as hemoglobin to separate oxygen from other gases. The chromoproteins are kept moist or in solution and are absorbed on or bound to filter paper; an electrolyte such as sodium chloride may also be present. The filter paper is alternately exposed to air (the carrier absorbs oxygen) and vacuum, which removes the bound oxygen.
Bonaventura, et al., U.S. Pat. No. 4,427,416 and 4,343,715, also disclose the use of naturally occurring oxygen carriers to extract oxygen from fluids. The metallo-proteins are insolubilized at high concentrations by entrapment and/or covalent linkage to a polyurethane matrix or similar, flexible support in states that are capable of reversibly binding oxygen. The material disclosed in these patents, generally known as "hemosponge" since it usually incorporates hemoglobin or another heme-type protein, is capable of extracting oxygen from various fluid environments. However, the rate of extraction is less than that which may be desired for many applications which involve a high rate of oxygen use. Further, these disclosures utilize chemical regeneration of the oxidized carrier compounds. with, e.g., ferricyanide solutions, which, in applications which require large amounts of oxygen, present considerable supply and waste disposal problems. Release of bound oxygen from the "hemosponges" requires either chemical oxidation of the carrier compound, With the concomitant supply and waste disposal problems, or various methods for pressing the hemosponge, which require pumps, vacuums, and the like which use substantial quantities of energy.
A variety of transition metal complexes with mono- bi-, and multi-dentate ligands are also capable of reversibly binding oxygen. Several devices and methods utilizing such synthetic transition metal oxygen-carrier compounds have been devised for extraction of oxygen from air. For example, Warne, et al., U.S. Pat. No. 2,217,850, discloses the reaction of oxygen in air with solids of cobaltous hexamine salts to synthesize, on a large scale, peroxocobalt amine solids, followed by removal of the solution, and separate chemical regeneration of the oxygen and the starting cobalt hexamine salts. Fogler, et al., U.S. Pat. No. 2,450,276, utilizes a solid cobaltous compound of a tetradentate Schiff base ligand to extract oxygen from air by alternately cooling a bed of the solid carrier compound, which absorbs oxygen from the air, and heating the oxygenated carrier compound to release bound oxygen. This process is accompanied by severe decomposition of the carrier compound. Iles, et al., U.S. Pat. No. 4,165,972, discloses an apparatus for alternately heating and cooling alternate beds of carrier compound to absorb oxygen from air into cooled beds of carrier and expel oxygen into a second gas handling system by heating the bed of carrier compound.
Solutions of transition metal carrier compounds have been used in both electrochemical and nonelectrochemical methods of extracting oxygen from air. For example. Gagne. U.S. Pat. No. 4,475,994, discussed above, utilizes cobaltous compounds to transport electrochemically-generated superoxide anions from the cell cathode to the anode where the oxygen is regenerated.
Roman, U.S. Pat. No. 4,542,010, discloses a method for producing oxygen and nitrogen using a porous, hydrophilic membrane support containing a solution of a transition metal oxygen carrier in a non-aqueous solvent. This device serves as a facilitated diffusion membrane; oxygen bound to the carrier diffuses from a first permeable membrane contacting air to a second membrane where the oxygen is released from the carrier. Thus, the permeability of oxygen through the membrane is increased by the reversible binding of oxygen to the organometallic carrier compound. Loading and unloading of oxygen from the liquid membrane is accomplished by a combination of temperature and/or pressure differentials. One drawback to this process is that oxygen generated using this device is costly, since the temperature and/or pressure differentials required to load and unload the oxygen carriers require large energy inputs. In addition, both sides of the membranes must remain saturated with solvent in order for the membrane to function, significantly adding to the cost and complexity of the device.
It is now well understood that many such transition metal-based carriers typically have a lower valence state. i.e., Mn(II), Fe(II), Co(II) or Cu(I), in which the carrier is capable of reversibly binding molecular oxygen under appropriate conditions, and a higher valence (more oxidized) state. e.g., Mn(III), Fe(III), Co(III), or Cu(II), in which binding of molecular oxygen is essentially absent. Most of the known methods for extracting oxygen from air using such transition metal carrier compounds are dependant upon the carrier compound remaining in the lower valence state. Molecular oxygen is absorbed from sources with a relatively high concentration (and hence chemical activity) of oxygen and reversibly bound to the carrier compound. The oxygen desorbs when the carrier compound is exposed to an environment in which the chemical activity of oxygen is lower, e.g., low oxygen partial pressures or elevated temperatures. Extraction processes may be carried out by exposing the carrier compounds to alternating environments of higher and lower oxygen activity, e.g., alternating partial pressure of oxygen or alternating low and high temperatures. The carrier compound may actually be used to carry oxygen from the feedstock environment to the delivery environment by diffusion or by pumped circulation.
After prolonged use, as with repeated heating and cooling cycles in bed reactors or oxygenated solutions, the carrier compounds tend to oxidize and become inactive toward oxygen. In solution, this oxidation and degradation often occurs via the formation of the stable and unreactive .mu. oxo dimer of, e.g., iron porphyrins. E.g., Leal, et al., J. Am. Chem. Soc. 97, 5125 (1975). Similarly, many cobalt-based carrier compounds tend to oxidize spontaneously to forms which are unreactive to oxygen. Various solutions to the problem of carrier compound decomposition have been suggested, although generally not with a view to oxygen extraction. For example, in attempts to protect the transition metal from its environment but allow diffusion of oxygen to the metal, various bulky ligands have been used. An alternate approach, suggested by Leal, et al., supra, is to immobilize the carrier compound on a solid support. Another approach to avoiding many of these difficulties is the regeneration of the lower valence state through the use of chemical and/or electrochemical reduction. Chemical regeneration methods may succeed, but, for the large scale production of oxygen from air, require large quantities of regenerating chemicals and typically produce commensurate quantities of waste chemicals.
Bonaventura, et al., in U.S. Pat. Nos. 4,602,987, 4,609,383, and 4,629,544, disclose another apparatus and methodology for extraction of oxygen from gas or liquid streams and delivery of the oxygen in high purity, in which the binding of oxygen to the transition metal carrier is modulated electrochemically. These patents disclose circulating a fluid containing dissolved organometallic carrier compounds to a region in which the carrier complex, in its lower valence state, is contacted with oxygen diffusing through a permeable membrane. The fluid, with oxygen bound to the carrier, is pumped to one electrode compartment in which the carrier is oxidized to its higher valence state, forcing the release of the bound oxygen. The carrier is then circulated to a second electrode compartment in which the carrier is reduced to its lower state in which it can again bind oxygen. Since the oxygen capacity of the fluid is enhanced by the solubility of the organometallic carrier compound, high partial pressures of oxygen may be obtained and oxygen may be "pumped" to concentrations or pressures greater than those of the feedstock fluid. The pressures of oxygen which may be produced are thus greater than the concentrations obtainable by simple diffusion through membranes or facilitated diffusion without electro-chemical unloading of the carrier compounds.
Artificial transition metal oxygen carriers which are potentially usable in oxygen extraction systems have been described by a number of researchers. For example, Brault, et al., Biochemistry 13, 4591 (1974), discloses the preparation and properties of ferrous deutero- and tetraphenyl-porphyrins in various organic solvents. Castro. Bioinorganic Chemistry 4, 45-65 (1974), discloses the synthesis of hexa- and penta-coordinate iron porphyrins, which are models for the prosthetic groups of active sites of certain cytochromes and other heme proteins. Other iron-containing transition metal compounds which may reversibly bind oxygen are described by Chang et al., J. Am. Chem. Soc. 95, 5810 (1973).
Numerous cobalt, manganese, and copper compounds also exhibit reversible oxygen binding. For example. Crumbliss, et al., Science 164, 1168-1170 (1969), discloses Schiff base complexes of Co(II) which form stable complexes with oxygen species in solution. See also, Crumbliss, J. Am. Chem. Soc. 92, 55 (1970) (monomeric cobalt complexes of oxygen); Dufour, et al., J. Mol. Catalysis 7, 277 (1980) (catalysis of oxidation of simple alkyl-substituted indoles by Co(II), Co(III), and Mn(III) meso-tetraphenyl porphyrins via a ternary porphyrin-indole-oxygen complex); Traylor, et al., J. Am. Chem. Soc. 96, 5597 (1974) (effect of solvent polarity on reversible oxygenation of several heme complexes prepared by reduction with sodium dithionite or a mixture of palladium black and calcium hydride); Held, U.S. Pat. No. 4,442,297 (absorption of gases using manganese compounds); Simmons, et al., J. Chem. Soc. Dalton Trans. 18-37 (1980) (reversible coordination of oxygen to copper (I) complexes of imidazole derivatives).
Some types of such transition metal carrier complexes have been used in or suggested for use in devices for extraction, absorption, and generation of oxygen from fluid media. For example, Roman, U.S. Pat. Nos. 4,451,270 and 4,542,010, discloses Schiff base complexes of metals in an oxygen-selective, permeable membrane and extraction system. The carriers include cobalt complexes of linear and macrocyclic tetradentate, linear pentadentate, and bidentate Schiff base ligands in primarily non-aqueous, Lewis base solvents. Hill, U.S. Pat. No. 4,442,297, uses phosphine complexes of Mn(II) in dehydrated solvents to purify nitrogen gas by extracting impurities including molecular oxygen. Sievers, U.S. Pat. No. 4,514,522, discloses oxygen sorbents comprising linear tetradentate ketoamine complexes bound to porous polymers. Gagne, U.S. Pat. No. 4,475,994, uses cobalt complexes of unknown stoichiometry in a mixed solvent at high pH to transport electrochemically-generated superoxide ions across a fluid membrane. Bonaventura, et al., U.S. Patents Nos. 4,602,987, 4,609,383 and 4,629,544, disclose a variety of metalloporphyrins, in combination with Lewis bases, in aqueous, non-aqueous, and water-immiscible solvents and their use to electrochemically separate oxygen from fluids.
Oxygen carrier compounds, including cobalt complexes of some linear, pentadentate polyamines, and their properties have been extensively reviewed and tabulated. Niederhoffer, et al., Chem. Rev. 84, 137-203 (1984). More detailed investigations of cobalt complexes of some linear, pentadentate polyamines have been reported in a series of articles by Harris, et al., and Timmons, et al. Inorg. Chem. 17, 889 (1978); Inorg. Chem. 17, 2192 (1978); Inorg. Chem. 18, 1042 (1979); Inorg. Chem. 18, 2977 (1979); Inorg. Chem. 19, 21 (1980); and Inorg. Chem. 21, 1525 (1982). The use of transition metal complexes of polyalkylamines in electrochemical or other oxygen extraction and generation processes is not known.